Recently, due to emerging environmental issues such as a global warming, a natural gas is becoming highly regarded in that the natural gas exhibits a higher hydrogen/carbon ratio as compared to other hydrocarbon fuels, a coal or the like. Therefore, natural gas can abate emissions of carbon-dioxide being an causative agent of the global warming, and the natural gas has abundant reserves. This, the demand for the natural gas is expected to increase in the future. Under such circumstances, there are a number of small and middle gas fields found in the regions of Southeast Asia, Oceania, etc., which however are still left undeveloped due to their locations of distant places having no infrastructure such as a pipeline and an LNG plant. This may require a significant amount of investment for the infrastructure being incomparable to their minable reserves, so that their developments have been desired to be processed. As one effective development effort, researches and developments for a technology, in which the natural gas is converted into a syngas and then the syngas is converted into a liquid hydrocarbon fuel (such as kerosene and gas oil having excellent transportability and handling characteristics by making use of a Fischer-Tropsch synthesis reaction), are promoted aggressively in various places.
This Fischer-Tropsch synthesis reaction converting the syngas into hydrocarbon with a catalyst is an exothermic reaction, where it is important to effectively remove reaction heat for a stable operation of the plant. As time-proven reaction processes, there are gas-phase synthesis processes (in a fixed bed, entrained bed, or fluid bed) and a liquid-phase synthesis process (in a slurry bed) having respective features. Recently, the liquid-phase synthesis process carried out in the slurry bed is more visible, and is being researched and developed strenuously for the reason that it exhibits a higher heat removing efficiency but avoids the accumulation of generated high-boiling point hydrocarbon on the catalyst as well as a reactor tube plugging caused thereby.
Generally, a higher catalytic activity is preferable, and especially, in the case of the slurry bed. There is a constraint that the concentration of slurry may be needed to be a prescribed value or below so as to keep a favorable slurry state, so that the increase in the catalytic activity is an important factor to increase a process design flexibility. The reported catalytic activities of various types of catalysts for the Fischer-Tropsch synthesis are approximately 1 (kg−hydrocarbon/kg−catalyst·hour) at most in view of the production rate of liquid hydrocarbon of a carbon number of five or above, which cannot be said always enough from the above-described viewpoints, as described in R. Oukaci et al., “Applied Catalysis A”, General, 186 (1999), p. 129-144, the entire disclosure of which is incorporated herein by reference.
As a method for improving the catalytic activity, a reduction in sodium content in silica used as a catalyst support has been described as being is effective—described in J. Chen et al., Cuihua Xuebao, Vol. 21 (2000), p. 169-171, the entire disclosure of which is incorporated herein by reference. However, the comparison was made only between the silica of the sodium content below 0.01 mass % and that of the sodium content of approximately 0.3 mass %, and there is no specific description as to the highest sodium content level started to be effected.
Further, generally, the particle diameter of the catalyst for the Fischer-Tropsch synthesis reaction is preferably provided as small as practically as possible from the aspect of reducing a possibility in which the diffusions of heat and matters come to a rate-determining level. However, in the case of the Fischer-Tropsch synthesis reaction in the slurry bed, out of the generated hydrocarbon, the high-boiling point hydrocarbon is accumulated in the reactor, inevitably requiring a solid-liquid separating operation for separating a product from the catalyst, so that there may be another problem that the catalyst of a too small particle diameter reduces the efficiency of the separating operation. Therefore, for the catalyst for the slurry bed, there should be an optimum particle diameter range, and generally, the range from about 20 μm to about 250 μm, or about 40 μm to about 150 μm as an average particle diameter, is considered to be desirable. However, as discussed below, there may be a case where the catalyst is caused to be fractured and pulverized to have a smaller particle diameter in the course of the reaction, requiring a caution.
In particular, in the Fischer-Tropsch synthesis reaction in the slurry bed, the operation can be frequently performed at an extremely high material-gas superficial velocity (>0.1 m/second), so that the catalyst particles clash furiously with each other during the reaction to possibly reduce their particle diameters during the reaction when the physical strength and abrasion resistance (resistance to be pulverized) are insufficient. This may at times cause an inconvenience in the separating operation. Further, in the Fischer-Tropsch synthesis reaction, volumes of water can be generated as a by-product. However, in the case of using the catalyst with low water resistance, which deteriorates in strength to be fractured and pulverized with ease due to water, the particle diameter of the catalyst is possibly reduced into a fine powder during the reaction, causing sometimes the inconvenience in the separating operation in the same manner as above.
As described above, the current catalytic activity may not yet be sufficient, and the catalyst with a higher catalytic activity has been requested as a pressing need, also from a viewpoint of extending the design flexibility in the plant.
Furthermore, generally, the catalyst for the slurry bed can be frequently put into practical use there by being prepared through a size control procedure by way of a grinding to have an appropriate particle diameter as described above. However, such a catalyst of a ground type may frequently have a crack or sharp protrusion arisen originally, and can effectuate a lesser mechanical strength and abrasion resistance. Thus, there may be a problem that the catalyst is forced to fracture to generate fine powders, and it becomes difficult to separate the generated high-boiling point hydrocarbon from the catalyst when used in the Fischer-Tropsch synthesis reaction in the slurry bed. Similarly, it is known that a relatively highly-active catalyst can be obtained when a porous silica is used as the catalyst support for the Fischer-Tropsch synthesis reaction. However, the size control based on the grinding may also lead to the strength deterioration due to the previously-described reason. In addition, the silica has lesser water resistance, and is frequently fractured into powders when water exists, thus causing problems especially in the case of the slurry bed.